Field of the Invention
The present invention relates to a detection method of a transmission laser beam for detecting, when a laser beam of a wavelength having a transparency to a workpiece is irradiated upon the workpiece, the laser beam having passed through the workpiece.
Description of the Related Art
A processing method is known wherein a laser beam of a wavelength having a transparency to a plate-shaped workpiece such as a semiconductor wafer or an optical device wafer is irradiated upon the workpiece with a focal point thereof positioned in the inside of the workpiece to form a modified layer, which serves as a start point of break, in the inside of the workpiece and then external force is applied to the workpiece to divide the workpiece into individual chips. In this processing method, although almost all of the laser beam is absorbed in the inside of the workpiece, so-called leak light appears which passes to a second face of the workpiece on the opposite side to an irradiation face (first face) of the workpiece by the laser beam (for example, refer to Japanese Patent Laid-Open No. 2012-59989).
A laser beam is oscillated by a laser oscillator and irradiated upon a workpiece through an optical system, which is configured from various optical parts, and a condenser. However, depending upon setting of the optical system, a cross section of a laser beam sometimes exhibits a distorted intensity distribution asymmetric in the radial direction. If such a laser beam as just described is used to process a workpiece, then a modified layer which is less liable to break may be formed or the appearance range of leak light may expand to such a degree that the leak light protrudes from a scheduled division line and has a bad influence on a device.